1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary cell, and more particularly to an improvement of a material to be used for a positive electrode thereof.
2. Prior Art
In association with a recent remarkable progress of electronic techniques, various portable electronic devices have been developed such as a camera-attached Video Tape Recorder, a pocket telephone, and a lap top computer. There is therefore an increasing demand for portable power sources for such electronic devices, i.e. a strong demand for development of a small-size secondary cell or battery having a higher energy density than the conventional ones. The widely known conventional secondary cells are: nickel-cadmium cells,lead accumulators, and lithium secondary cells.
The lithium secondary cells are expected as cells capable of composing a battery having a high power with a high energy density. Some of the lithium secondary cells are already available in a market. Because lithium has strong reactivity against water, such lithium secondary cells employs lithium salt such as LiPF.sub.6 dissolved in a non-aqueous solvent such as propylene carbonate and diethyl carbonate.
The lithium secondary cell may employ a metallic lithium or a lithium alloy as a negative electrode active material. Recently, however, there is a tendency to employ a material which will not produce a dendrite even charge and discharge are repeated, for example, such as a carbonaceous material which is capable of doping and de-doping lithium ions.
As for a positive electrode active material, a metallic sulfide, and a metallic oxide such as TiS.sub.2, MoS.sub.2, NbSe.sub.2, and V.sub.2 O.sub.5 having no lithium have been used. Recently, however, more and more lithium-nickel composite oxides are used because of a higher energy density and a lower production cost.
It should be noted that a lithium secondary cell using such a lithium-nickel composite oxide for the positive electrode active material has a problem that a charge capacity during an initial charge is greater than a discharge capacity by 40 to 50 mAh/g. Consequently, the ratio between the lithium which has been separated during charge and the lithium which has been returned, i.e., charge/discharge property (initial charge capacity/initial discharge capacity) is lower by 5 to 6% than that of a lithium-cobalt composite oxide which is used in practice. The low charge/discharge property affects the energy density and has been a serious obstacle for realizing a large capacity.